Coal is transformed into coke when it is baked in the absence of air in refractory ovens arranged in batteries. The transformation process releases a large volume of hot gases (approximately 700-750° C.) at the beginning of the baking phase that gradually decreases as the process advances.
In each oven, these gases are discharged through a riser pipe closed by a valve at the top. Each riser pipe opens, via a bent discharge pipe, into a barrel, which is a large-diameter pipe that collects the gases from the oven battery and routes them to a treatment circuit. Each bent discharge pipe comprises an ammonia water nozzle to cool the gases to about 80° C. before they enter the barrel and a sealing plate (or plate), located at the intersection with the barrel, and which can take two positions. In the open position, the plate is vertical, leaving the passage entirely free for the gases. In the closed position, the plate is horizontal, enabling to completely stop the passage of the gases towards the barrel.
During the coking phase, the riser pipe valve of each oven is closed, enabling the evacuation of gases to the barrel. The sealing plates of the ovens are in the open position at this time.
When coking is completed and the coke is unloaded out of the oven, the valve is opened and the oven plate is closed, which enables to isolate the barrel and to evacuate the residual gases through the riser pipe.
In most coking plants, only the pressure on the barrel is regulated around a set point that varies between 70 and 150 Pa depending on the height of the ovens. At the beginning of the baking phase, the high gas pressures in the ovens result in the emission of gas through the oven doors, despite the care taken in their maintenance and their leakproofness. These emissions, which comprise fine particulates, carbon monoxide and polycyclic aromatic hydrocarbons (PAH) have a significant impact on working conditions and the quality of ambient air.
At the end of baking, the ovens may be under negative pressure and air from the outside may be let in, eventually resulting in the deterioration of the refractory bricks due to combustion close to the oven entrances.
Some coking plants that give priority to the life cycle of ovens work with a high barrel pressure to maintain positive pressure in ovens at the end of the baking, even if this creates emissions at the doors. Other coking plants, which are subject to very stringent environmental requirements, such as the United States with the Clean Air Act, operate at very low barrel pressures, at the risk of deteriorating the oven entrances and adversely impacting the oven life cycle.
To reduce smoke emissions during loading, there is a first type of coking plant wherein the ammonia water nozzle placed in the bent discharge pipe projects pressurized water (30 to 40 bars) in the direction of the flow of gases, thus creating a suction effect of these gases to the treatment circuit. This nozzle operates at high pressure for only two minutes when the coal is loaded into the ovens and during the five minutes following this loading, whereas baking lasts between 16 and 20 hours. If a high barrel pressure is maintained, this technology does not enable emissions to be eliminated during the first hours of baking of the ovens.
To improve the management of these gas emissions, there is a second type of coking plant in which the pressure in each oven can be controlled separately. It is described in particular in WO 02/094966. This plant comprises a bent discharge pipe extended inside the barrel by a straight pipe with crenel cutouts through which the gases pass. This pipe leads to a dome filled with water in the manner of a siphon, the dome being connected to a pneumatic cylinder that is used to adjust the height of the dome and hence the surface of passage of the gases. The pneumatic cylinder is controlled depending on the pressure measurements taken inside the oven, and the barrel is maintained under suction throughout the oven baking cycle. This system allows the oven pressure to be regulated efficiently and, in particular, to maintain a slight positive pressure at the end of baking.
However, it has the disadvantage of being mechanically complex, thus requiring significant maintenance. It also requires a substantial modification of existing facilities. Lastly, all the equipment must be doubled for safety reasons and its use does not prevent gas leakage at the oven doors when coking begins.
There is also a last type of coking plant where the pressure in each oven can be controlled separately. This system consists in changing the first type of plants by articulating and controlling the position of the oven plate, so it can take all the possible intermediate positions between the open vertical position and the closed horizontal position. It is thus possible to regulate the pressure in each oven by partially closing the plate according to the pressure measured at the bottom of each riser pipe. The plate is also modified to take the form of a bell on its upper part, in order to optimize the system sensitivity, especially when it is closed completely. However, this system has some disadvantages in practice, because it is very sensitive to the slightest change in pressure and requires fine control, which is difficult to implement.